The modification of plants by genetic engineering has lagged behind the understanding and utilization of the molecular biology of unicellular organisms and mammalian cells. Techniques that have proven effective for stable transformation of unicellular microorganisms or mammalian cells with foreign DNA have not found useful analogy with plant cells. Therefore, despite the many achievements involved with unicellular microorganisms and mammalian cells, the number of achievements with plant cells has been substantially fewer and the experience with the other types of organisms has not been readily translatable into successful practices with plant cells.
In many situations it will be desirable to modify an existing trait of a plant cell, rather than introduce a new trait. Thus, one may wish to modify the activity of a particular enzyme, provide for the preferential expression of one allele as compared to another, one isozyme as compared to another, or the like. In many instances one may only wish to reduce the amount of expression of a structural gene, rather than inhibit expression entirely. It is therefore of interest to develop techniques which will allow for directed modification of the phenotype of particular plant cells, plant tissues or plants.